The present invention relates to an anti-lock control method and apparatus for preventing locking of automotive wheels of a running vehicle.
Generally, in an anti-lock control apparatus for automotive vehicles, to maintain steering ability and running stability during braking, a control unit including a microcomputer controls brake hydraulic pressure. This brake pressure control also reduces braking distance.
During operation of the anti-lock control apparatus, a control mode for brake hydraulic pressure is determined based on an electric signal indicative of a wheel speed. This wheel speed is detected e a wheel speed sensor. According to the control mode, a hold valve, which is a normally opened solenoid valve, and a decay valve which is a normally closed solenoid valve, are selectively opened or closed to increase, hold constant and decrease the brake hydraulic pressure.
In such anti-lock control, as disclosed in the U.S. Pat. No. 4,984,164, a reference speed for deciding when to decrease the brake hydraulic pressure is set or determined in accordance with a wheel speed (herein after referred as system speed Vs) to be controlled in each brake control system. There is also set an estimated vehicle speed Vv having a predetermined follow-up limit of acceleration/deceleration with respect to the highest wheel speed among four wheel speeds. Further, there is set a threshold speed which tracks the estimated vehicle speed Vv with a predetermined speed difference therefrom and below the estimated vehicle speed Vv.
Based on the comparison of the system speed Vs with the reference speed or the threshold speed and also on the detected high peak point and low peak point of the system speed Vs, a pressure decrease status, a pressure hold status and a pressure increase status are set. Then, a plurality of hydraulic pressure valves (solenoid valves) are turned on and off in accordance with a predetermined control mode set for each of these statuses, thereby effecting the pressure decrease, pressure hold and pressure increase.
The above-mentioned control method, however, suffers from a problem in that the control mode remains fixed or unchanged until the status is changed, even when the wheel speed is abruptly changed due to variations in the road condition. Thus, the method fails to react in a timely manner which is particularly disadvantageous because the wheels may lock during abrupt braking. Another disadvantage is that since the pressure increase is effected at the time when the wheel speed recovers to a speed near the vehicle speed, the wheel speed is repeatedly accelerated and decelerated in a speed range having a certain slip rate with respect to the vehicle speed. As a result, the brake hydraulic pressure is increased and decreased causing a vibration of the vehicle body.